Solid-state image intensifier with variable contrast ratio



April 18, 1957 TADAo KoHAsl-u A.

SOLID-STATE IMAGE INTENSIFIER WITH VARIABLE CoNTRAsT RATIO 3 Sheets-Sheet l Filed Nov. 18, 1963 Zgw'ehyf 72 de@ /f /9 4 g/z/ /f ATTORNEYS April 18, 1967 TADAo KoHAsHl SOLID-STATE IMAGE INTENSIFIER WITH VARIABLE CONTRAST RATIO Filed Nov. 18. 1963 3 Sheets-Sheet 2 ATTORNEYS April 18, 1967 TADAO KOHASHI 3,315,080`

SOLID-STATE IMAGE INTENSIFIER WITH VARIABLE CONTRAST RATIO Filed Nov. 18, 1963 3 Sheets-Sheet 5 In Venbr ATTORNEYS United States Patent O 3,315,080 SOLID-STATE IMAGE INTENSHFIER WITH VARIABLE CNTRAST RATIO Tadao Kohashi, Yokohama, Japan, assignor to Matsushita Electric Industrial Co., Ltd., Osaka, `Iapan, a corporation of Japan Filed Nov. 18, 1963, Ser. No.` 324,483 Claims priority, application Japan, Nov. 20, 1962, 37/ 52,254 2 Claims. (Cl. Z50- 213) The present invention relates to solid-state image intensiers and particularly to those of the type comprised principally of three elements including an electroluminescent element, a photoconductive element and an electrical impedance element insulated from each other and have respective necessary electrodes, said principal elements forming a composite solid plate, if required, with at least one auxiliary element, said principal elements being also connected with respective electrical power sources and arranged electrically and in'space in a manner so that the electroluminescent element is influenced by the photoconductive and electrical impedance elements so as to exhibit on the electroluminescent element a visible light signal or image as transduced or intensified in accordance with lthe incident radiation signal or image upon the photoconductive element.

A solid-state image intensier of the type described has previously been known in which alternating-current vector voltages V1 and V2 are impressed respectively across the electroluminescent and photoconductive elements and U across the electroluminescent and electrical impedance elements, said voltages V1 and V2 being selected to be opposite in phase or to have `a phase difference therebetween of 180 with a bridge connection formed to reduce the luminescence of the electroluminescent element caused by the dark current thereby to enlarge the contrast ratio of the luminescence intensity of the electroluminescent element so that a positive visible image is obtained for the incident radiation image.

With such known intensifier, an enlarged contrast ratio has been obtainable bu-t it has been impossible to make variable the behaviour of the operating characteristics and particularly of the 'y characteristic. In other words, the previously obtained characteristic of the input radiation versus the output light has been of the remote type.

In general, radiation signals or images used in solidstate image intensifiers of the type concerned are generally diverse and it is desirable or rather critically important that the operating characteristics are variable over a wide range in order that the contrast ratio or contrast ('y value) of the output image may be freely magnifled or attenuated and that the range of ltransducing or intensifying the incident image can be restricted by selection of the sharp and remote cutoff characteristics.

The present invention has for its object to provide a solid-state image intensifier of the type described having two operating characteristics of different behaviours and in which the ranges of variation or selection of the absolute values of said two alternating-current vector voltages are restricted, said vector voltages having one and the same selected frequency and having a controlled phase difference therebetween in combination with variation in the alternating-current electrical impedance characteristic associated with the intensity of incident radiation upon the photoconductive element thereby to render the luminescence intensity characteristic of the electroluminescent element variable in a wide range in association with the radiation intensity upon the photoconductive element.

According to the present invention, there is provided a solid-state image intensifier of the type described characterized in that alternating-current vector voltages V1 and V2 of the same frequency are fed from respective power sources having a variable phase difference to be impressed across said electroluminescent and photoconductive elements and across said electroluminescent and electrical impedance elements, respectively, the former voltage V1 having an absolute value selected in a range in which said electroluminescent element produces a luminescence of a satisfactory intensity by an alternating current associated with the voltage at least under the condition of a maximum conductivity of said photoconductive element while the latter voltage V2 having an absolute value selected in a range not higher than the critical absolute value corresponding to the point of minimum luminescence intensity on the characteristic curve of the absolute value of V2 versus the luminescence intensity of the electroluminescent element under the condition that the voltage V1 has a xed absolute value when the phase difference is fixed at the critical value 61 which corresponds to the point of minimum luminescence intensity on the characteristic curve of the phase difference 9 between said voltages V1 and V2 versus luminescence intensity of said electroluminescent element as obtained with the absolute values of said voltages fixed under the condition that the photoconductive element has a minimum conductivity, it being arranged such that said phase difference 9 between said alternating-current vector voltages V1 and V2 or the absolute value in said range of at least one of said voltages V1 and V2 is controlled or made variable thereby to control the luminescence of the electroluminescence element in an alternating-current fashion.

In this specification, it is to be understood that any of the elements may take the form of a wire, ribbon, film, laminate or any other suitable form. The term electroluminescent element as used herein refers generally to a solid cold luminescent element which luminesces at least by an alternating current at an intensity variable with the alternating-current electrical field or electrical current. The term radiation refers to any radiation such as visible or invisible light, X-rays, a-rays, 'y-rays, -rays or an electron beam. The term photoconductive element generally refers 'to any element formed of solid substance at least the electrical conductivity or resistance of which varies upon reception of such radiation.

The term electrical impedance element usually refers to any element formed of solid substance exhibiting a capacitive, resistive or intermediate impedance characteristic to an alternating-current power source, but in this specification such element may be formed of any solid or liquid substance as long as it is of the character described.

The present invention will now be described in detail with reference to the accompanying drawings, which illustrate some preferred embodiments of the invention, each of which have given satisfactory results in experiments conducted by the inventor.

In the drawings:

FIG. 1 illustrates the variable operation system according to the present invention including one form of solidstate image amplifier shown in longitudinal cross section and the power supply system therefor;

FIG. 2 is a performance chart showing the experimental results obtained with the embodiment shown in FIG. 1 and employing the phase difference, G, as a parameter;

FIG. 3 is chart experimentally obtained with the embodiment of FIG. 1 and showing the relationship between the luminescence output at dark, [L2]L1=0, and the phase difference, 9, between V1 and V2;

FIG. 4 is a chart experimentally obtained with the embodiment of FIG. l and showing the relationship between the luminescence output at dark, [L2]L1=0, and the absolute voltage value V20 when 9:90;

FIG. 5 is a schematic diagram of one practical application of the variable operation system and one form of power supply system for the solid-state image intensifier according to the present invention having the same construction as the one shown in FIG. 1;

FIG. 6 is a circuit diagram of the inventive variable operation system according to the present invention including a form of variable phase shifter for the solidstate image intensifier; and

FIG. 7 is a circuit diagram showing the inventive variable operation system and a form of power device for supplying alternating-current vector voltages V1 and V2 to the solid-state image intensifier.

Reference is first made to FIG. l, in which there is illustrated a system employing visible light as radiation and the visible light image is intensified to form a positive green image on an electroluminescent element. In this example, all the elements are arranged in lamination. Relative dimensions of parts in FIG. l are shown in conveniently enlarged scales but it will be apparent that the present invention is not to be limited to such relative dimensions.

In FIG. l, there is shown a support plate 1 of transparent glass on one side of which is deposited a lighttransmitting electrode 2 of tin oxide. On the electrode 2 various elements are laminated in the order described below.

The electroluminescence element 3 has a thickness of approximately 60a and is formed of a mixture of an epoxy resin with an electroluminescent powder of ZnS 2 CuAl effective manner and also to prevent breakdown of the insulation between the above electrode 2 and a grid electrode 7 to be described below. The insulation element 4 also has a high dielectric constant compared to the electroluminescence element 3 for effective electric excitation of the element 3 and has a thickness of approximately 20a, being formed of a mixture of an epoxy resin with a fine powder of white, highly light-reiiecting dielectric material having a high dielectric constant, BaTiO3, to meet the requirement of reducing the electric impedance.

The non-light-transmitting dielectric element 5 is a layer of black organic resin paint having a high specific resistance and a thickness of the order of approximately p. Effecting light interception, the element 5 serves to prevent any light feedback due to the luminescence output of element 3 and the exterior light coming from the outside of electrode 2 from making the operation unstable and also to prevent any input radiation signal or image coming through an electrode 9 to be described ybelow from passing through the element 5 to be superimposed on the luminescence of the element 3 and thus deteriorate the image quality.

Incidentally, the element 5 can be omitted in the event that the element 6 is irresponsive to visible light or the luminescence of the element 3 and that the input radiation is invisible. Also, in case the light feedback from element 3 is positively utilized in controlling the operation characteristics the element 5 may partially limit the light feedback or be translucent or be omitted.

The photoconducting element 6 is a layer formed of a mixture of a photoconductive powder of CdSrCuCl with an epoxy resin as a binder and having a thickness of approximately 60u. The element is highly sensitive to visible light and near infrared rays and may be formed of any selected material depending upon the kind of the incident radiation, however, is preferably formed -of a material having a low dark conductivity and a high light- Jto-dark conductivity ratio. In the illustrated example, the material used has the ratio of 104 or over.

The grid electrode 7 is intended to supply electric power to the photoconducting element 6 and may take any form having open spaces. For example, it may be comprised of parallel metal wires, braided metal wire netting, or a solid netting electrode of metallic material as one employed in image tubes. The open spaces in the element also serve as a passage of the exciting current for element 3. The grid electrode 7 is preferably bonded to or at least partially embedded in -that surface of the photoconducting element 6 remote from the element 3.

In the illustrated example, the electrode 7 takes the form of a parallel metallic wire electrode including gilded tungsten Wires of circular cross section having 10p. thickness in order to minimize the coverage of the electrode 7 to electrode 2 and also to obtain an improved electrical contact with the element 6. The tungsten wires are arranged at intervals of 30G/t to enhance the resolving power for the image in compromise `with the coverage referred to above and are embedded in that surface of the element 6 remote from the element 3 so as to be partially exposed on the surface to utilize the high photoconductivity of the input radiation surface which is excited directly by the input radiation. The electrode 7 is also welded to an electrically conducting strip 11 on the end surface of said support plate l opposite to the electrode 2.

The electrical impedance element 8 in this example is formed of a radiation transmitting substance and takes the form of a transparent polyester film in view of the use of an incident radiation including Visible light or near infrared rays. This film involves only an extremely small dielectric loss exhibiting a substantially pure capacitive impedance. The element 8 has a thickness of approximately 50p.

The electrode 9 associated with the electrode 8 is covered with a support plate of transparent glass 10 which transmits the incident radiation. The light-transmitting capacitive impedance element S is laminated between the electrode 9 and the photoconducting element including electrode 7 by use of an adhesive medium such as silicone oil convenient in-experimentation. For industrial applications, however, a light-transmitting organic resin adhesive is reeommendable in place of silicone oil for solid lamination.

The radiation-transmitting electrode 9 in this instance takes the form of a light-transmitting electrode, for example, of tin oxide like electrode 2 since the incident radiation used includes visible light or near infrared rays. However, where the input radiation includes X-rays or like radiation rays, the electrode 9 may take the form of a vapor-deposited metallic film or a thin metal sheet.

The elements described above are laminated togetherV to form a composite solid plate for the radiation transformer and image intensifier.

Reference numeral 12 designates a sine-wave oscillator and its electrical signal output is applied to continuous variable phase Shifters 13 and 14 including a combination of resistance, capacitance and other electrical elements with vacuum tubes for the purpose of regulating the phase difference between the two sine-wave signals. 16 designate respective electrical signal amplifiers, each of which permits elimination of either one of said phase Shifters 13 or 14. In this case, the ysine-wave electrical signal used on that side having no phase shifter takes the very form of the output signal of oscillator 12. T1 and T2 are transformers for the 4output of said respective signal amplifiers. The outputs of the transformers, i.e., the alternating-current vector voltages V1 and V2 from the secondary sides thereof are applied to the above composite solid-plate.

More specifically, transformers T1 and T2 each have one of their output terminals conected to a common conductor 16a and therethrough to light-transmitting electrode 2 associated with the electrolumineseence element 3. The other output terminal of transformer T1 is connected by way of a conductor 17 to the electrically conductive strip 11 to apply the sine-wave alternating-current vect-0r voltage V1 between the grid electrode 7 associated with the 15 and 5 photoconductive element 6 and the light-transmissive electrode 2.

The other output terminal of the transformer T2 is connected by way of a conductor 18 to light-transmitting electrode 9 associated with light-transmissive capacitive impedance element 8 to apply the sine-wave alternatingcurrent vector voltage V2 of the same frequency as said V1 between the electrode 9 and said light-transmitting electrode 2.

For convenience in description, the voltages V1 and V2 are selected in accordance with light-transmitting electrode 2 and the phase difference between the voltages is considered upon the basis of the voltage V1 though the two voltages V1 and V2 are periodic waves. V2 is ahead in phase of V1 and the phase voltages will be expressed herein 'clS V1=V10 wf and V2=V20 (Lj-G).

In these formulae, V10 and V20 represent the absolute values of the respective voltages, w represents the angular velocity, indicating that the phase of V2 is in advance of that of V1 by the angle 9.

The luminescene output L2 of the electroluminescence element 3'is controlled by the absolutevalue, I2I, of the sum, I2, of vector currents I1 and I2, which flow through the element in association with said vect-or voltages V1 and V2 ([I'3[=l1-l-2|). The current I1 is a photo current caused to ow through the grid electrode 7 under V1 in association with the change in conductivity in the surface of the photoconductive element 6 and in the direction of its thickness as induced by the input light L1, which passes through support plate 10, electrode 9 and electrical impedance element 8 onto the photoconductive element 6. The photo current can be expressed by the formula 1=I10 (wf-idx).

The other current 2 is a through current caused to flow under V2 through the electroluminescent element 3 by way of impedance element 8, photo-conductive element 6 and` elements and 4, and can be expressed by the formula I'2=I20 Sin (ar-po-i-a).

The phase difference relationship is such that the current l2 is evaluated as a leading phase current with respect to current I1, and the phase lead is expressed by p=9t+-u- All of these pha-se differences should be determined in radian units, but for convenience in description they are expressed in degrees hereinafter.

FIG. 1 illustrates the case where the phase difference 9 between V1 and V2 and that (p between 1 and I2 are both 180. The indication of currents 1 and l2 in FIG. l is only for convenience and does not accurately correspond to their real paths.

To describe the operation of the illustrated example, the absolute value V10 is selected within a range in which the electroluminescent element 3 produces a sutiicient luminescence by the current I1 yat least under the conditions for the maximum electrical conductivity of the photoconductive element 6. It is to be understood that such absolute value V10 determines the brightness of the output-indicating visible image.

In cases where, as in the illustrated example, a visible image is to be formed which is transformed or intensified relative to the input radiation image, the element 3 is selected so as to produce a satisfactorily high luminescence under the conditions that the input light L1 is large enough and the element 6 is in a high conductive state.

In FIG. 2 there is illustrated the relationship between the input light L1 and the output light L2 when in the ex-ample of FIG. l t-he input light L1 is a filtered light from a tungsten-filament lamp having a predominant Wavelength of 648 ma and a half-value Width of 22 ma and V10=300 volts, V1 having a fixed frequency of f=800 c.p.s. The characteristics represented by dotted lines in FIG. 2 are those obtained when V2 is opened and the element 3 is excited in an alternating-current fashion by the current I1 alone. It will be clearly observed that a satisfactorily high luminescence is obtainable in the high region of L1, that is, under the condition of high electrical conductivity.

On the other hand, the voltage V2 has the same frequency as V1 and its absolute value, V20, can be determined assuming that the respective absolute values of V1 and V2 are iixed under the condition of the minimum electrical conductivity of the photoconductive element 6. The selection is made such that, when the phase difference 0 between V1 and V2 is fixed -at the critical value 90, which corresponds to the point of minimum intensity of luminescence on the characteristic curve of the phase difference G versus the intensity of luminescence of the electroluminescent element 3, the absolute value V20 of V2 corresponds to or is smaller than its critical value, which corresponds to the point of minimum intensity of luminescence on the characteristic curve of the absolute `value V20 versus the intensity of luminescence L2 of the electroluminescent element 3.

In this example, where a visible output image is to be obtained from an input radiation image, the photoconductive element 6 has a minimum electrical conductivity when the input light L1='0. In FIG. 3 is illustrated the relationship experimentally obtained between the phase difference 9 between V1 and V2 and the dark the explanation. The luminescence, [L2]1,1:0, is continuous variable phase Shifters 13 and 14 while adjusting means 12, 13, 14, 15 and 6 to maint-ain the input light L1=0, V10=300 volts, V20=450 volts and f=800 c.p.s. The value of V20 corresponds to its critical value V200, which allows the maximum range of variation in the operation characteristic, just for the purpose of simplifying to give a minimum value of {l2} and hence of [L2]1,1=0. trollable by the absolute value of the resultant current, [13| |I1-l-I2] and both I1 and I2 are a capacitive alternating-current vector current maintaining the relationship of ot, since the currents I1 and I2 are substantially purely capacitive with the extremely small conductivity of the photoconductive element 6 and the electrical impedance element 8 is also capacitive involving only an extreme small loss similarly to the elements 3, 4 and 5, which also are capacitive with a minimized loss. Thus, the currents are leading in phase relative to 4the voltages V1 and V2, establishing the relationship of p=9|-a29- On the other hand, the absolute value of the luminescence current, [I3|=|I1+I2l, which can be obtained vectorically, can be varied by varying 9, and, when 02180? and hence p=l3+5a=180, the currents I1 and I2 act to cancel each other to the greatest extent to give a minimum value of [l2] and hence of [L2]L1:0. The cancelling effect is reduced as the deviation of G from 180 land when 9:0 or 360 the currents act most additively to give a maximum value of |10' and hence of [L2]L1:0. Therefore, as observed in FIG. 3, the characteristic curve of [L2]L1:0 versus phase difference G, which determines the contrast ratio of the visible output image, is generally V-shaped for the range of 9 varying from 0 to 360 under adjustment of the phase shifters 13 and 14. In other words, [L2]1,1=0 is of a minimum value at 0:180" and has a value increasing on opposite sides in a symmetrical fashion so 'that it s controllable over a substantial range. It s :also observed that the phase difference has a critical value of :180".

FIG. 4 illustrates the characteristic of V20 versus [L2]1,1:0, which is obtained at 9:90:180", V10=300 volts and f=800 c.p.s. by measuring V20 while adjusting the signal amplifier 16. As V20 increases, I1 is cancelled by I2 to an increasing extent to give a minimum of lIal and hence of [L2]L1:11 at V20=450 volts, which is therefore the critical absolute value V211c of the voltage V2. Thus, in the present invention, V20 is limited to a range of V20V20C- In this region, it is indicated that the resultant luminescent current I3 of electroluminescent element 3 is dominated by I1 or else 1131 is at a minimum when V211=V20C. However, the range of V20 V20c where I2 is dominant, is inapplicable with the present invention. It is particularly pointed out that the light out1 put, [L2]L1:0, when V20=V211c=450 v, has only a value of the order of 11176 of the value obtained when V2 is opened as referred to hereinbeforc. In this manner, the critical phase difference 0c=l80 and the critical absolute value V2M- 1450 volts have been experimentally determined for V-:300 volts.

Under these conditions, the phase difference 0 is changed according to the present invention to vary the operation characteristics of the device over a wide range.

In FIG. 2 is illustrated the relationship between t-he input light L1 and output light L2 obtained by adjusting and fixing the phase shifters 13 and 14 to give a phase difference of 9:0", 90, 135, 180, 225, 270 and 360.

As observed in this figure, increase in L1 increases the electrical conductivity of the photoconductive element 6 in the direction of its surface as well as of its thickness. In accordance with this, the absolute value of I1, i.e., 1111 is increased and the electrical impedance of the element 6 grows increasingly resistive in nature to decrease a thereby causing a change from a capacitive to a resistive vector current. On the other hand, current I2 is not influenced to any substantial effect as it is a through current and the element 6 is suitably thin, whereas a is reduced slightly and 122 tends to increase slightly but this increase is small compared to the change of I1. Therefore, the relationship between the phase difference G and the phase difference p related with V1, V2 and L1 is given by 6 or p=01o when a radiation L1 exists. This relationship is intensified with increase in L1 at least over the lower range of L1.

Assume now that the device is operated over a range of 6 of 180 9 360. When dark, there exists a relationship -of 9= p, as described above, and [L2`j1,1;1 which determines the contrast ratio of t-he visible output image, is defined as illustrated in FIG. 3 relative to the phase difference between V1 and V2, as illustrated in FIG. 3.

When radiation L1 is given, 110 is increased whereas a is reduced, and thus it is apparent that p is increased at all times at a rate higher that 9 under the condition of 9 p=9+#a. In this region, therefore, p tends to go far off 180 at least over the lower region of L1.

Now, if the range of evaluating the absolute value of the phase difference p between I1 and l2 is restricted to the range of 0180 for evaluating p by substitu tion of therefor, will decrease with increase in L1 at least over its lower range. This indicates that I1 and I2 are at all times vectorially additive and hence that L2 exhibits a monotonous increasing characteristic over the lower region of L1, giving the device a remote cutoff lpositive image amplifying characteristic.

In contrast to this, for the region of 0 9 l80, p or approaches to 180 at least over the lower portion of the region of L1 as L1 and hence 110 increase. This causes an operation effective to vectorially cancel the increase in llgl, which results from increase in 110.

Additionally, the decrease of :p is pronounced in the lower region of L1 since the photoconduetive element 6 is capacitive when dark. Accordingly, in this region L2 does not increase with increase in L1 at a rate as high as in the preceding case, but remains constant or rather t decreases slightly. However, as L1 increases further, increases at a substantial rate while p decreases at a lower rate so that [Isl is controlled predominantly by increase in-l11 -characteristically exhibiting a monotonous increase with L1 to give the device a sharp cutoff positive image amplifying characteristic. In the higher region of L1 or under the condition of the maximum conductivity of the photoconductive element 6, 110 is much larger than and the latter is negligible so that L2 has a substantially fixed value irrespective of the value of 9.

Therefore, as clearly observed in FIG. 2, when 9:90:180" and hence p==180, the contrast 'y has a value of 2 giving an extremely high contrast ratio of the order of 2 l04. Gn the other hand, when 9=O or 360 and hence p==0, fy has a reduced value of 0.55 giving a contrast ratio of the order of as low as 2 10. In the intermediate region of 9, i.e., when 0 180, the contrast ratio is continuously variable in a symmetrical fashion in accordance with the deviation from 60:9:180, while fy is variable asymmetrically over a wide range of 0.55'y2. In addition, in the region of l809 360 where decreases with increase in L1, an operation behaviour is obtainable which is quite different from the remote cutoff characteristic, and, in the region of 09 180, where increases with increase in L1, a behaviour quite different from the sharp cutoff characteristic is obtainable.

It will be appreciated, therefore, that in transforming and intensifying the positive visible image corresponding to an input image the contrast ratio and 'y can be freely amplified or attenuated simply by varying G on the same panel. In addition, one and the same contrast ratio can give characteristics having different Iy and one of which can produce an accurate output image over the entire range of localized intensity distribution of the input image and the other acts to Cut Off the lower intensity region while displaying the intermediate and higher intensity regions.

Particularly in the latter case, where a sharp cutoff characteristic is obtained, a clear positive image can be pr-oduced, unwanted signals including noises in the lower region of the input image being eliminated. This makes it possible, for example, in multistage type positive-image amplifiers to obtain a satisfactory multistage positiveimage amplification, which has previously been almost impossible due to amplification of the luminescence output at dark causing saturation at the final stage to reduce the contrast ratio. Thus, according to the present invention a positive image amplifier device can be obtained which can afford many different operations previously unobtainable.

Further, as observed in FIG. 2, these two different operation characteristics can each be varied by varying 9, and the range of adjustment of G can be restricted depending upon the purpose so as to exhibit a remote cutoff characteristic, including decrease in with L1, or alternatively to give a sharp cutoff characteristic. In these cases, G need not always be adjusted continuously, but may be adjusted advantageously to one or more different values in practical applications, by use of resistance, capacitance and other phase Shifters, as shown in FIG. 1, or by discontinuous switching operations.

Through the operation characteristic for V20=V211c is most desirable having a maximum range of control, as described above, substantially the same characteristic is obtainable by selecting the voltage V20 V20c- In this case, the range of the contrast ratio is reduced, but particular convenience is obtained for fine adjustment of ry and the contrast ratio as well as the sharp or remote cutoff characteristic. The ymode of operation is desirable also in that, where an appropriate fixed value of 9 is selected and V211 has a range of variation of 0 V20V2W it is possible to make variable or control the fy, contrast lratio and behaviour of the operation characteristic.

Though in the above explanation of the experimental value of c is 270,

example the electrical impedance element used is substantially capacitive, it may be replaced by an element differing in nature, for example, a resistive element or an element exhibiting a characteristic intermediate resistive and capacitive. The Value 9c, which gives gp==180, varies with the electrical impedance of the element used. Namely, if the element is of the resistive nature, t-he limit Q0 generally ranging over 180 0c 270 In the illustrated example, the photoconductive element 6 used has an impedance which is nearly pure capacitive When dark. As the present invention utilizes the change of (or zp, which relates to a) with L1, it is preferable to use `material which affords a wide range of the change in the since the usual forms of photoconductive element 6 and electrical impedance element 8 Arange from a pure capacitive to a pure resistive form.

The above various ranges are applicable even when an electroluminesceut element 3, and auxiliary elements such as 4 andS are formed of material which has a resistive or intermediate impedance characteristic since they form together an impedance common to I1 and 2, though in the above these elements have been described as exhibiting a capacitive impedance. Also, though the respective elements have been described as electrically linear elements photoconductive and electroluminescent elements generally have a nonlinear electrical characteristic, and I1, I2 each have a distorted waveform. However, the same generalization applies by employing the phase dierence between the distorted waves if they have the same tendency or by applying the phase difference between the respective basic waves if the distorted Waves are different from each other, since the principal effect is produced by the basic wave components.

Since 9 is generally a periodic function having a period of 360, it will be understood that the sharp cutoff operation characteristic is obtained in the range of 0 S go 180 If 9 is limited in association with lthe critical phase difference Gc to the range of 91,-1809 9cl180 for relative estimation, the sharp cutoff operation characteristic is obtained in the range of 9c--180"9 9c since p=180 at 9:96. On the other hand, it is found that the remote `cutoff operation characteristic obtains in the range of q: of 180 p 360 and hence in the range of G of Gc9 6+l80- In this case, both (9c-180) and (6c+180) are apparently a periodic function having a period of 360 and quite the same in phase. If

(GC-180) has a negative value indicating that V2 is estimated as vhaving a phase lag with respect to V1. Similarly, if 9c 180, (9d-180) exceeds 360 indicating that V2 is estimated with respect to V1 as a leading phase exceeding 360.

Though in the above description has been made in conjunction with the arrangement of FIG. 1, the present invention is not to be limited to the details set forth but is applicable to any solid-state image intensifier based upon the principles of the invention.

Thus, the present invention is not restricted to image amplification but may include independent cells or other separate units forming photoconductive, electrical irnpedance and electroluminescence elements, respectively, which can be connected by conductors to form a required electrical circuit for exciting the photoconductive element by means of an input radiation signal thereby to obtain on the electroluminescent element a visible signal corresponding to the radiation signal. In this case, the electrical impedance element imay take the form of an inductive impedance such as a coil Reference will next be made to FIG. 5, which diagrammatically illustrates the structure of another form of solidstate image intensifier embodying the present invention and the power supply system therefor. In this ligure, reference numerals like those in FIG. l designate parts like those disclosed therein. The illustrated structure generally indicated by 20 is based on quite the same principles as the one shown in FIG. l. This embodiment has an important feature that it employs a simplified operation system for obtaining a remote cutoff characteristic as described hereinbefore and including a changeover switch 19 therefor, and a power supply system adapted to vary the operation characteristic without the need of changing V1 and V2.

If a as in the case of FIG. 1 where the electrical impedance element 8 is purely capacitive and the photoconductive element 6 has a capacitive dank impedance, the behaviour of the operation characteristic varies with the contrast ratio substantially held constant around Therefore, in such case, the remote and sharp cutoff operation characteristics can be freely selected by switch ing yoperation between V1 and V2 or between shifted electrical signals for supplying V1 and V2. Thus, the inconvenience that 9 must be controlled to establish another behaving operation characteristic corresponding to the sharp or remote cutoff operation eliminated in this embodiment.

The rapid switching from the remote cutoff operation characteristic to the sharp cutoff characteristic or vice versa during the input image radiation is very advantageous in various applications, for instance, in investigating the nature of the input image. In one method of changing 9 in this manner, a switch 19 is provided as shown in FIG. 5 to select the electrical signals for V1 and V2. By this means, the behaviour of ,8 relative to L1 is completely in correspondence as far as e is concerned. As a result, the need for controlling the values of V1 and V2 is eliminated by setting the electrical output signals of the continuous variable phase Shifters I3 and 14 both at the same value and arranging so that V111 and V211 are adjusted by the electrical signal amplifiers I5 and I6.

Of course, in this case the switch 19 may be arranged in the input or output circuits of an appropriate stage in the amplifiers I5 and I6. Alternatively', such switch may be arranged in the secondary side of T1 and T2 for direct switching between V1 and V2 while adjusting V111 and V20 by ampliers I5 and 16. Such switching system is highly desirable particularly in case the phase Shifters I3 and I4 are comprised of one or more simple resistive or capacitive phase shifter circuits to be selected freely by switching operation, since a wide range of variable 9 can be obtained even with a limited number of such circuits.

In other words, the phase Shifters I3 and I4 in FIG. 5 and the number of the circuits described above are preferably selected so that the variable range of G covers a half-period range bounded by Gc. In this manner, a range corresponding to one complete period can be covered by the switch operation to enable all possible operations. It will be apparent that such arrangement is very desirable from the economical standpoint and also for ease of operation.

In the structure indicated at 20, the alternate sets of wires of the grid electrode 7 are insulated from each lll yother and connected to respective conductive strips 11A and 11B for applying a direct-current variable voltage VB across the two sets of the electrode wires, which are connected to T1 through respective alternating current bypassing direct current blocking capacitors C1 and C2 to receive V1. Generally, photoconductive elements formed of a mixture of a photoconductive powder such as of CdS with a plastic or other dielectric binder material is subject to deterioration in alternating current photoconductivity and must be recovered by superimposing thereon a direct-current electrical field. The degree of such recovering is a function of VB. As illustrated, therefore, the photoconductive element is arranged to form a closed direct current circuit (also including a pulsating current) and an alternating current voltage V1 is applied so that the circuit together with the electroluminescent element forms a series closed circuit. With this arrangement the element can be driven in an alternating-current fashion to control the alternating current photoconductivity of the photoconductive element while keeping the luminescence of the electroluminescence element free from any effect associated with VB. Accordingly, the value of L1 can be shifted by controlling V13 while maintaining the operation characteristic curves as shown in FIG. 2 in a state parallel over a range of approximately two places with V1 and V2 xed. Thus, the operation characteristics can be varied over a wider range under the above behaviour conditions. Particularly in the case of the sharp cutoff characteristic, the L1 region to be cut may be varied affording an advantage that the characteristics of the input radiation image can readily be investigated in detail.

Further, this system can be applicable on the same principle as described in cases where the photoconductive elements take the form of cells or by associating a plurality of such photoconductive elements with an electroluminescent element in an electric circuit. In cases where a plurality of photoconductive elements which are each a discontinuous element are arranged, the same principle as described above can be applied between the adjacent photoconductive elements.

FIG. 6 illustrates a variable phase shifter arrangement embodying the present invention which is applicable to the above described continuous variable phase shifters 13 and M. With the variable operation system and the solid-state image intensifier according to the present invention, it is required in controlling the phase difference between V1 and V2 to minimize wave distortion occurring in the course of phase-shifting the respective electrical signals and also to minimize the change in V13 and V20 due to such phase shifting. Substantial distortion or change in V13 and V20, if any, adversely affects the range of variable operation and complicates the operation of the device to an extreme extent. This embodiment is designed to provide a fully satisfactory variable phase shifter arrangement which is free from these diiculties.

In FIG. 6, reference numerals 22 and 24 each indicate a conventional four-element four-terminal variable phase shifter circuit, which includes a pair of resistances R3 and R4 preferably of the same value arranged in one pair of opposite circuit arms and interconnected so as to be varied in unison. A pair of capacitors C3 and C4 are arranged in the other pair of opposite arms and preferably have .the same capacitance. With this circuit, the phase can be continuously varied theoretically over a half pcriod of to 180 by varying R3 and R4 from zero to an infinite value and the phase-shifted output signal obtained from an input electrical signal may have a fixed absolute value. On this occasion, however, the input impedance taken from the input end toward the input electrical signal is required to be sufficiently small compared to the impedance value between the two input terminals of the phase-shifter circuit due to change in R3 and R4 while the out impedance value taken from the other independent units,

two terminals from which the phase-shifted electrical signals are extracted must be extremely high compared to the impedance between the two input terminals.

Moreover, with this circuit, amplification or multiplestage connection of the phase-shifted electrical signals is infeasible since it is a four-terminal circuit which cannot have any unified connecting system unlike ordinary twoterminal circuits, which employ either a balanced or unbalanced connection. Additionally, it is extremely difficul-t to obtain with this four-terminal circuit electrical signals free from any substantial distortion since the control of R3 and R4 causes a substantial change in the impedance value and hence the impedance value of the load, for example, including an electronic amplifier or transducer unit.

In this embodiment, the above problems are solved in the following manner. As illustrated, the input terminals comprising a pair of opposite terminals are balance-connected to the secondary coils of the output side of vacuum tube transformer load cathode followers 21 (T3), 23 (T4). Further, the load impedance value of the output side of the transformer output cathode follower and the impedance value taken from the input end of the phase-shifter circuit toward transformers T3 and T4 are set at values lower than the minimum allowable impedance of the four-terminal phase-shifter circuit by arranging a dummy load RD, for example, as shown, while on the other hand connecting a pair of output terminals out of balance to the input circuit of the vacuum tube cathode follower circuit.

This cathode follower system lprovides a perfect feedback effect improving the signal distortion and with the input signal fed through the output end of the cathode follower circuit enables minimization of said input irnpedance value so that the variation in the absolute values of the four-terminal circuit d-ue to impedance change is highly improved. Moreover, since the output terminals of the four-terminal circuit are connected to the input terminals of the cathode follower circuit, any adverse effect of the output impedance can be completely eliminated at extremely high impedance values.

In the illustration, these phase-shifter units include a two-stage cascade connection but a single stage or more than two stage cascade connection may be employed therein as required.

In addition, the resistance load cathode follower circuit including vacuum tube 25 is a buffer unit for taking out the electrical signal displaced in phase at the final stage and is also connected to the input circuit of the cathode follower circuit to increase the output impedance value taken from the output terminals of the phase shifter circuit as described hereinbefore. Thus, an electrical signal E1 is obtained which is shifted in phase by @c with respect to the input signal 1, and is connected to electrical signal amplifier 15 or switch 19 shown in FIG. 5. Next, experimental researches will be described demonstrating the usefulness of the phase shifter in this illustration.

The specific circuit shown in FIG. 6 includes a cascade connection of three phase Shifters made to the same specifications. In these phase Shifters, C3=C4=0.02p.f., R3 and R4 forming a dual variable resistor having a range of from zero to 200KQ. The sine wave signal used has a frequency of 800 c.p.s. The minimum impedance value of the four-terminal phase shifter circuit is about 5K9 at R3=R420- The dummy load RD is a resistance RD=0.6KS2. The turn ratio of output transformers such as T3 `and T4 is 110.66. The vacuum tubes 21, 23 have a load impedance v-alue of about 1.3Kt2. The buffer device is similar to the one shown in FIG. 6 including vacuum tube 25, and employs vacuum tubes of the 12AT7 type. As shown, an input signal 1 in the form of a sine wave signal voltage 1111 is shifted in phase to obtain a shifted output signal voltage 1. The deviation of the absolute value E of the output voltage 1' when the phase G is shifted over a range of one period of from -to 360 was within $10.3 db giving a distortion of 1% or less.

Though in the above example of FIG. 6, R3 and R4 was changed in a continuous fashion for continuous phase-shifting, the same operation is obtainable even with fixed R3 and R4 by interconnecting C3 and C4 to form a gang capacitor. Further, the phase shifting may be effected in Ia discontinuously fashion if desired by employing a plurality of either capacitor or resistance elements arranged for selective use by switch means with the other limpedance elements having a fixed value. Also, though in the above example C3 and C4 were given a fixed value of 0.02nf.,they may each include a plurality of capacitors arranged for switching operation therebetween to have an adjustable capacitance for the purpose of accomplishing a similar phase-shifting operation over a wide frequency range.

FIG. 7 illustrates one form of the invention means for supplying at least one of the alternating-current vector voltages V1 and V2. In FIG. '7, 26 indicates a power -amplifying vacuum tube and T5 a step-up transformer, which is connected to the cathode side of 26 to form a step-up transformer output cathode follower vacuum tube circuit in which input voltage signal V0 is raised and an output Voltage appears across the secondary coil terminals a and b.

As described in detail with reference to FIG. 1, according to the present invention, alternating-current vector voltages such as V1 and V2 are impressed to obtain the desired operation. As will be understood, the voltages are required to have a minimum of distortion and any change is operation in their absolute values and in the phase difference therebetween should be avoided as far as possible.

On the other hand, -as described hereinbefore in connection with FIG. l, the photoconductive element 6 gives a current I1 which `increases with incident radiation L1 and has an impedance value variable over a substantial range. The element 6 corresponds in structure to lan electrical capacitor having a substantial capacitance and necessitates an ineffective current of a substantial magnitude having a limited impedance value.

Under these circumstances, it is considerably difficult for the power supply means to satisfy the above conditions by use of an ordinary power amplified. In other Words, the limited impedance necessitates a high rating device which is comparable thereto. Also, a high output voltage is required because of the relatively high values of V10 and V20. Thus, the power supply means necessarily has a substantial size under limitations from the internal impedance of the power source. In addition, the impedance varies over a considerably wide range with increase in the incident light L1 rendering the load impedance unmatched therewith. This apparently causes change in absolute value of the output voltage, increases the rate of distortion and necessarily causes a phase deviation rendering the operation of the system unstable. These difficulties can be overcome by use of the means of FIG. 7. In this instance, since the internal impedance in the cathode follower output power source is extremely limited, the voltage may readily be raised to a value of about seven times as high by use of step-up transformer T3 without increasing the distortion rate. Also, because of the limited internal impedance, the fluctuation of the output Voltage value with increase in L1 is extremely small. Further advantages include a very limited phase deviation and distortion rate due to the perfect negative feedback effect. Particularly, in cases where a dummy load RD is arranged in the secondary side as shown and has a selected resistance value lower than the impedance value determined by the structure of the intensifier device and the output impedance has a properly selected value, all ofthe adverse effects described above can be eliminated completely. Also, the electrical signal shifted in phase by use of the continuous variable phase shifter means as described hereinbefore is amplified in voltage and impressed as V0 and the output voltage formed across terminals a and b is supplied as V1 or V2, as described hereinbefore in connection with FIG. l.

With this example, as long as it has a structure as shown in FIG. l having an area of' the order of 10X10 cm2, a satisfactory operation can be attained by use of vacuum tubes having an output rating of the order of 10 watts. Thus, a compact system which is stable in operation and economical can be obtained without use of any high power transmitting tubes or the like elements conventionally regarded as indispensible.. The step-up transformer T5 may have its primary and secondary coils connected in series with each other to serve as a kind of step-up autotransformer. In this case, the voltage formed across the primary coil and the output voltage developing across the secondary coil are added to each other to further increase the step-up ratio of the transformer.

Though in the above the photoconductive element has been described as having a conductivity increasing With the intensity of the incident radiation to produce an intensiiied or transformed positive visible signal or image corresponding to the input radiation signal or image, a negative visible signal or image may also be obtained by a similar transforming or intensifying procedure. This objective can be attained by using a photoconductive element formed of a material having a conductivity which increases upon reception of a visible light, ultraviolet rays, a-rays, X-rays or like radiation, but decreases upon reception of infrared rays or like radiation., One known example of such material is CdS activated by silver with gallium used as an auxiliary activator. For example, the photoconductive element 6 in FIG. 1 may be made of such material and irradiated with ultra-violet rays, fit-rays, X-rays, visible light or like radiation of a uniform or nearly uniform intensity to obtain an increased conductivity and hence a satisfactory luminescense of the electroluminescent element 3. Under this situation, if an infrared signal or image is radiated superimposingly upon the photoconductive element, the conductivity of the latter will be reduced according to the local intensity of the infrared radiation to reduce the luminescence of the electroluminescent element 3. It will thus be observed that the transformation in this case can be made into a negative signal or image.

Also, in this case, the photoconductive element 6 exhibits a maximum conductivity when no radiation of an infrared signal or image to be intensified is given and a minimum conductivity at a maximum local intensity of the infrared signal or image. Therefore, a sharp and remote cutoff characteristics are given in a region where the infrared radiation has a higher intensity or the electroluminescent element 3 has a lower luminescence output so that those portions of the infrared signal or imagehaving a higher infrared radiation intensity can be cut or accurately displayed. The operation in this case is quite the same as in the case of the positive operation except for the negativeness of the behaviours such as the contrast ratio and In addition, in the instant case, the operation characteristics can be controlled over a wider range by regulating the intensity of auxiliary radiation such as described hereinbefore with the infrared signal or image held in a fixed state.

It is to be understood that the present invention is not to be restricted to the details set forth but many changes and modifications may be made without departing from the spirit and scope of the present invention.

What is claimed is:

1. A solid-state image intensifier comprising a composite solid plate including an auxiliary element and three principal elements, said principal elements consisting of an electroluminescent element, a photoconductive element and an electrical impedance element, positioned in a sandwich relationship, each of said principal elements being insulated from each other and having electrodes connected thereto, electrical power means connected to said electrodes whereby the electroluminescent element influenced by the photoconductive and electrical impedance element exhibits a visible light signal in accordance with an incident radiation signal applied to the photoconductive element, said electrical power means providing alternating current vector voltages V1 and V2 having the same frequency, variable phase shifting means for feeding said alternating current vector voltages V1 and V2 of the same frequency at a variable phase difference across said electroluminescent and photoconductive elements and across said electroluminescent and electrical impedance elements respectfully, said electrical power means providing said V1 voltage with an absolute value in a range whereby said electroluminescent element produces a luminescence of satisfactory intensity in response to the alternating current associated with said voltage at least under the condition of maximum conductivity of said photoconductive element, said electrical power means further providing said V2 voltage with an absolute value corresponding to a point of maximum luminescent intensity on a characteristic curve of the absolute value of V2 versus the luminescent intensity of the electroluminescent element in which the voltage V1 has a fixed absolute value when the phase difference between voltages V1 and V2 is xed at a critical value Qc, which corresponds to the point of minimum luminescent intensity on the characteristic curve of the phase difference 9 between said voltages V1 and V2 versus the luminescent intensity of said electrolumines-cent element, as obtained with the absolute values of said voltages xed with the photoconductor element at a minimum conductivity, whereby said variable phase shifting means provides means for varying the light transducing characteristic of said electroluminescent element, said variable phase shifting means comprising a plurality of four-element four-terminal phase-shifter circuits in cascade connection, each of said circuits having a pair of variable resistances arranged in one pair of opposite arms of the circuit forming a gang resistor, and a pair of capacitors arranged in the other pair of opposite arms of the circuit; and vacuum-tube transformer load cathode followers, each having a secondary coil on the output side to which a pair of opposite terminals of said phase-shifter circuit are balance-connected, and resistor means connected in parallel with said secondary coil whereby the value of the load impedan-ce on the secondary coil side of said vacuum-tube transformer output cathode follower is lower than the minimum impedance value of the associated fourterminal phase-shifter circuit, and the other pair of opposite terminals of each of said four-terminal phase-shifter circuits is connected out of balance to the input circuit of an associated vacuum-tube cathode follower.

2. A solid-state image intensifier comprising a composite solid plate including an auxiliary element and three 1b principal elements, said principal elements consisting of an electroluminescent element, a photoconductive element and an electrical impedance element, positioned in a sandwich relationship, each of said principal elements being insulated from each other and having electrodes connected thereto, electrical power means connected to said electrodes whereby the electroluminescent element influenced by the photoconductive and electrical impedance elements exhibits a visible light signal in accordance with an incident radiation signal applied to the photoconductive clement, said electrical power means including at least one vacuum-tube step-up transformer output cathode follower circuit for providing alternating current vector voltages V1 and V2 having the same frequency, variable phase shifting means for feeding said alternating current vector voltages V1 and V2 of the same frequency at a variable phase difference across said electroluminescent and photoconductive elements and across said electroluminescent and electrical impedance elements respectively, said electrical power means providing said V1 voltage withian absolute value in a range whereby said electroluminescent element produces a luminescence of satisfactory intensity in response to the alternating current associated with said voltage, at least under the condition of maximum conductivity of said photoconductive element, said electrical power means further providing said V2 voltage with an absolute value corresponding to a point of maximum luminescent intensity on a characteristic curve of the absolute value of V2 versus the luminescent intensity of the electroluminescent element in which the voltage V1 has a xed absolute value when the phase difference between voltage V1 and V2 is fixed at a critical value 9c, which corresponds to the point of minimum luminescent intensity on the characteristic curve of the phase difference 9 between said voltages V1 and V2 versus the luminescent intensity of said electrolumines-cent element, as obtained with the absolute values of said voltages xed with the photoconductive element at a minimum conductivity, whereby said variable phase shifting means provides means for varying the light transducing characteristic of said electroluminescent element.

References Cited by the Examiner o RALPH G. NLSON, Primary Examiner.

M. A. LEAVTT, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,315 ,O80 April 18 1967 Tadao Kohashi It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as showm below:

Column l5, line 28, "maximum" should read minimum Column 16, line 29, "maximum" should read minimum ed and sealed this 7th day of April 1970.

Sign

(SEAL) Attest: Edward M. Fletcher, Ir. WILLIAM E. SCHUYLER, JB..

Commissioner of Patents Attesting Officer 

1. A SOLID-STATE IMAGE INTENSIFIER COMPRISING A COMPOSITE SOLID PLATE INCLUDING AN AUXILIARY ELEMENT AND THREE PRINCIPAL ELEMENTS, SAID PRINCIPAL ELEMENTS CONSISTING OF AN ELECTROLUMINESCENT ELEMENT, A PHOTOCONDUCTIVE ELEMENT AND AN ELECTRICAL IMPEDANCE ELEMENT, POSITIONED IN A SANDWICH RELATIONSHIP, EACH OF SAID PRINCIPAL ELEMENTS BEING INSULATED FROM EACH OTHER AND HAVING ELECTRODES CONNECTED THERETO, ELECTRICAL POWER MEANS CONNECTED TO SAID ELECTRODES WHEREBY THE ELECTROLUMINESCENT ELEMENT INFLUENCED BY THE PHOTOCONDUCTIVE AND ELECTRICAL IMPEDANCE ELEMENT EXHIBITS A VISIBLE LIGHT SIGNAL IN ACCORDANCE WITH AN INCIDENT RADIATION SIGNAL APPLIED TO THE PHOTOCONDUCTIVE ELEMENT, SAID ELECTRICAL POWER MEANS PROVIDING ALTERNATING CURRENT VECTOR VOLTAGES V1 AND V2 HAVING THE SAME FREQUENCY, VARIABLE PHASE SHIFTING MEANS FOR FEEDING SAID ALTERNATING CURRENT VECTOR VOLTAGES V1 AND V2 OF THE SAME FREQUENCY AT A VARIABLE PHASE DIFFERENCE ACROSS SAID ELECTROLUMINESCENT AND PHOTOCONDUCTIVE ELEMENTS AND ACROSS SAID ELECTROLUMINESCENT AND ELECTRICAL IMPEDANCE ELEMENTS RESPECTFULLY, SAID ELECTRICAL POWER MEANS PROVIDING SAID V1 VOLTAGE WITH AN ABSOLUTE VALUE IN A RANGE WHEREBY SAID ELECTROLUMINESCENT ELEMENT PRODUCES A LUMINESCENCE OF SATISFACTORY INTENSITY IN RESPONSE TO THE ALTERNATING CURRENT ASSOCIATED WITH SAID VOLTAGE AT LEAST UNDER THE CONDITION OF MAXIMUM CONDUCTIVITY OF SAID PHOTOCONDUCTIVE ELEMENT, SAID ELECTRICAL POWER MEANS FURTHER PROVIDING SAID V2 VOLTAGE WITH AN ABSOLUTE VALUE CORRESPONDING TO A POINT OF MAXIMUM LUMINESCENT INTENSITY ON A CHARACTERISTIC CURVE OF THE ABSOLUTE VALUE OF V2 VERSUS THE LUMINESCENT INTENSITY OF THE ELECTROLUMINESCENT ELEMENT IN WHICH THE VOLTAGE V1 HAS A FIXED ABSOLUTE VALUE WHEN THE PHASE DIFFERENCE BETWEEN VOLTAGES V1 AND V2 IS FIXED AT A CRITICAL VALUE 0C, WHICH CORRESPONDS TO THE POINT OF MINIMUM LUMINESCENT INTENSITY ON THE CHARACTERISTIC CURVE OF THE PHASE DIFFERENCE 0 BETWEEN SAID VOLTAGES V1 AND V2 VERSUS THE LUMINESCENT INTENSITY OF SAID ELECTROLUMINESCENT ELEMENT, AS OBTAINED WITH THE ABSOLUTE VALUES OF SAID VOLTAGES FIXED WITH THE PHOTOCONDUCTOR ELEMENT AT A MINIMUM CONDUCTIVITY, WHEREBY SAID VARIABLE PHASE SHIFTING MEANS PROVIDES MEANS FOR VARYING THE LIGHT TRANSDUCING CHARACTERISTIC OF SAID ELECTROLUMINESCENT ELEMENT, SAID VARIABLE PHASE SHIFTING MEANS COMPRISING A PLURALITY OF FOUR-ELEMENT FOUR-TERMINAL PHASE-SHIFTER CIRCUITS IN CASCADE CONNECTION, EACH OF SAID CIRCUITS HAVING A PAIR OF VARIABLE RESISTANCES ARRANGED IN ONE PAIR OF OPPOSITE ARMS OF THE CIRCUIT FORMING A GANG RESISTOR, AND A PAIR OF CAPACITORS ARRANED IN THE OTHER PAIR OF OPPOSITE ARMS OF THE CIRCUIT; AND VACUUM-TUBE TRANSFORMER LOAD CATHODE FOLLOWERS, EACH HAVING A SECONDARY COIL ON THE OUTPUT SIDE TO WHCIH A PAIR OF OPPOSITE TERMINALS OF SAID PHASE-SHIFTER CIRCUIT ARE BALANCE-CONNECTED, AND RESISTOR MEANS CONNECTED IN PARALLEL WITH SAID SECONDARY COIL WHEREBY THE VALUE OF THE LOAD IMPEDANCE ON THE SECONDARY COIL SIDE OF SAID VACUUM-TUBE TRANSFORMER OUTPUT CATHODE FOLLOWER IS LOWER THAN THE MINIMUM IMPEDANCE VALUE OF THE ASSOCIATED FOURTERMINAL PHASE-SHIFTER CIRCUIT, AND THE OTHER PAIR OF OPPOSITE TERMINALS OF EACH OF SAID FOUR-TERMINAL PHASE-SHIFTER CIRCUITS IS CONNECTED OUT OF BALANCE TO THE INPUT CIRCUIT OF AN ASSOCIATED VACUUM-TUBE CATHODE FOLLOWER. 